1. Field of the Invention
The present invention relates to a magnetic disk drive, and, more particularly, to a magnetic disk drive that ensures excellent fault tolerance by preventing an excessive rise in coil temperature of a voice coil motor (hereinafter referred to as “VCM”) even when there is frequent access of data to a magnetic disk at high temperature in the magnetic disk dive that performs the seek operation of a magnetic head by the VCM.
2. Description of the Related Art
To prevent the heating of a motor, various arts have been known before as a means for controlling a general motor. For example, the “motor controller” disclosed in the Japanese Patent Laid-Open No. Hei 11-341850 detects a current that is supplied to the motor and incorporates the current in the CPU, then compares the current with several reference temperatures. The rotational velocity of the motor is adjusted by adjusting the rotational speed of the motor based on the result, thereby preventing the heating of the motor.
In a general magnetic disk drive, a VCM has been used as a mechanism for moving a magnetic head to a predetermined track. A control means for preventing the heating of the motor must also be provided in this VCM when the magnetic disk drive is frequently accessed. The magnetic disk drive that uses the VCM introduces the following digital control system. That is, the magnetic disk drive reads position information recorded on the magnetic disk at equal time intervals in the circumferential direction to position the magnetic head for data access in a desired track and transfers the position information to the CPU, then performs predetermined operation. A D/A converter converts an operation result to an analog signal. A VCM AMP converts this voltage to a current and applies the current to the VCM.
Besides, the operation that moves a magnetic head to an object track is called “seek operation” and the state in which data read and write is being executed in an object track or remains stationary and is positioning is called “following operation” here.
A magnetic field is applied to the VCM by a magnet arranged in the vicinity in this manner. When the VCM AMP makes a current flow in the VCM for positioning operation, drive force that propels an actuator is generated by electromagnetic induction, thereby enabling the seek operation that moves the magnetic head to the predetermined track. At seek operation, a coil moves in a magnetic field, back electromotive force (hereinafter referred to as “BEMF”) that is proportional to the traveling velocity occurs.
The BEMF is calculated from (Equation 1).BEMF=Vvcm−Ivcm×Rvcm  (Equation 1)
Where,
Vvcm: VCM both-end voltage
Ivcm: Current flowing in a VCM
Rvcm: Coil resistance of a VCM
The BEMF can be calculated from the aforementioned (Equation 1) if the VCM voltage and the VCM current can be measured regarding the VCM coil resistance as a constant. However, the VCM coil resistance varies with the heat dissipation when the supply of a high VCM current by the seek operation and the ambient temperature of a magnetic disk drive. The VCM coil resistance is generally expressed by (Equation 2).Rvcm=R20×(1+C×Δt)  (Equation 2)
Where,
R20: VCM coil resistance when the VCM ambient temperature is 20° C.
C: Temperature sensitivity constant
Δt: Temperature difference of 20° C. with the VCM coil temperature
Because the VCM coil resistance has temperature sensitivity like this, a slight error occurs. If the VCM coil resistance (Rvcm) can be measured, however, Δt can be calculated back by transforming (Equation 2), thereby also enabling acquisition of the VCM temperature.
On the other hand, some magnetic disk drives have a load and unload mechanism that shunts a magnetic head to an area on the outer circumference than a disk surface if the power of the magnetic disk drives is not supplied when a spindle motor that rotates a disk is in a stopped state except in the read and write operation.
In this case, to enter a normal ready state from an unready state, the load operation in which an actuator is moved from the shunting position to the disk surface must be controlled. And to enter the unready state from the normal ready state, the unload operation in which the actuator is moved from the disk surface to the shunting position must be controlled, too. Positioning information are recorded on the disk surface. As described above, when the magnetic head is shunt from the disk surface, the positioning information cannot be read, thereby disabling positioning control, such as normal seek operation. Accordingly, velocity control is performed by detecting the BEMF that is proportional to the traveling velocity of the actuator expressed in (Equation 1). In such case, the magnetic disk drive is necessarily provided with a VCM current detection circuit and a VCM voltage detection circuit.
When the VCM coil resistance varies with a temperature change, the sensitivity of the BEMF to be detected differs. Accordingly, the VCM coil resistance must previously be learned. In this method, for example, as the art described in the Japanese Patent Laid-Open No. 2000-222837, the actuator is pushed against the external-surface stopper at the load operation and the VCM current is supplied. At that time, because the traveling velocity=0, it follows that BEMF=0. The VCM coil resistance can be learned by calculating the following equation in the transformation of (Equation 1).Rvcm=Vvcm/Ivcm  (Equation 3)
For the unload operation, the same measurement may be made by pushing the actuator against the internal-surface stopper.
Hereupon, the problem the present invention is attempting to solve is described below.
As described above, the VCM coil resistance has temperature sensitivity and varies with the seek operation and ambient temperature. Hereupon, the data access pattern of a magnetic disk drive differs in sequential access and random access, and there are various access data lengths in one command. In case of a VCM, the severest pattern is a short access data and the amount of VCM current required for the seek operation per hour is high. Specifically, in this case, deceleration current is supplied when the data length is 1 or 0 sector and seek operation reaches the specified maximum velocity continuing making the current that accelerates an actuator flow.
Hereupon, the relationship between the VCM current and the seek velocity at that time is shown in FIG. 7. FIG. 7 is a diagram showing the general relationship between the VCM current and the seek velocity.
When a command continues and is issued at high frequency in this manner, the VCM current continues being supplied and the VCM coil temperature will rise extremely. When the ambient temperature of the drive is high, the temperature will further be raised.
At the VCM coil temperature, a temperature change does not occur steeply in the one-time seek operation, but slowly changes by the execution of multiple times of seek operations. In general, a severe seek continues several 10 ms or more and a measurable temperature change appears. When this severe state further continues, the heat resistance temperature of a mechanism component is exceeded and the characteristics of a functional component vary, thereby causing a fault to occur. Concerning such symptom, for example, as described in the Japanese Patent Laid-Open No. Hei 6-119008, when a temperature sensor is provided inside the drive and a specified value is exceeded, there is a method of stopping operation.
The problem was, however, that, in actual, a real temperature and a measured value differ frequently according to the mounting location of the temperature sensor and this is not necessarily suitable for being used to prevent a fault.
When the VCM coil temperature rises, the coil resistance increases and the amount of acceleration current decreases during seek control, thereby decreasing acceleration capability. That the traveling capability of a mechanism system decreased is detected by measuring the traveling distance of the fixed section of an acceleration section using this characteristic. If this traveling distance is below a fixed value, a method for switching the seek control at low velocity and suppressing the heat dissipation of the VCM is also known.
However, this method may not be able to estimate the VCM coil temperature correctly because the acceleration capability varies with the power supply voltage, thereby fluctuating the power supply voltage.
As described in the Japanese Patent Laid-Open No. 2000-222837, a magnetic disk drive that performs measurement operation by pushing an actuator against the external-surface stopper or internal-surface stopper for the measurement of VCM coil resistance is known.
In the measurement of this VCM coil resistance, however, a considerable amount of measurement time including the traveling time from the current position to the stopper, measurement time when the actuator is pushed against the stopper, and the traveling time to the next command position are required. In this method, if the rise of the VCM coil temperature is set as a problem when a high frequency instruction continues, this command itself is suspended and the stopper pushing operation for measuring the coil resistance must be executed. Consequently, access performance will deteriorate considerably. Moreover, this Japanese Patent Laid-Open No. 2000-222837 also describes that the measurement of the VCM coil resistance is executed in a “following” state. In actual, a small amount of the VCM current can be found in the “following” state. When the method of A/D-converting the VCM current via the VCM current detection AMP is used, an A/D conversion value reaches an extremely small amount in comparison with the resolution. Accordingly, a measurement result with good accuracy is difficult to obtain.
The art described in the Japanese Patent Laid-Open No. Hei 11-341850 of the aforementioned prior art controls a motor in which the amount of current and the heating temperature of the motor are simply proportional, but there is a problem that this art is hard to apply to the VCM. This is because an object of the VCM is to position a head. Accordingly, when a current flows in a short time and will not flow immediately, a pattern is repeated periodically. As a result, the mere detection of the VCM current is difficult to couple with the cause of a motor temperature rise. In this point, it is expected that a VCM for a magnetic disk drive should be controlled in line with the actual circumstances when the traveling velocity for a fixed period is considered.
The present invention is made under such circumstances and it is therefore a future of the present invention to provide a magnetic disk drive having excellent fault tolerance that will not generate a fault even in such severe environment as the issue of a high humidity and high frequency command by estimating the VCM coil temperature itself and, as a result, moving to the seek control mode that suppresses an excessive temperature rise without using a special device, such as a temperature sensor, and requiring a special operation for the measurement of the VCM coil temperature and deteriorating access performance.